1. Field of the Invention
The present invention relates to a bias adjustment method and the like for an optical modulator.
2. Description of the Related Art
In a wavelength multiplexing optical communication system of the next generation, it is expected that a switch-over between wavelength channels is performed in a node. Therefore, a wavelength conversion apparatus is required in the wavelength multiplexing optical communication system. As a wavelength conversion apparatus, an optical single side-band (SSB) modulator is known. The optical SSB modulator is an optical modulator which can obtain an output light having shifted for a frequency of a modulating signal (e.g. as described in [S. Shimotsu, S. Oikawa, T. Saitou, N. Mitsugi, K. Kubodera, T. Kawanishi and M. Izutsu, “Single Side-Band Modulation Performance of a LiNbO3 Integrated Modulator Consisting of Four-Phase Modulator Waveguide,” IEEE Photon. Tech. Lett., Vol. 13, 364-366 (2001)] and [Shimotsu Shinichi, Masayuki Izutsu, “LiNbO3 optical SSB modulator for next-generation communication”, Optical Alliance, 2000.7. pp. 27-30]).
FIG. 4 is a schematic diagram showing a basic arrangement of an optical SSB modulator. As shown in FIG. 4, an optical SSB modulator 101 is provided with a first sub Mach-Zehnder waveguide (MZA) 102, a second sub Mach-Zehnder waveguide (MZB) 103 and a main Mach-Zehnder waveguide (MZC) 104.
In the optical SSB modulator 101, sinusoidal radio frequency (RF) signals (modulating signals) different from each other by a phase of 90° are inputted to four arms paralleled in MZA 102 and MZB 103. Also with respect to light, bias signals are inputted so that phase differences of the light in the four arms are respectively 90°. Then, a light whose frequency is shifted by the frequency of the modulating signal is outputted. The direction of frequency shifting, namely whether the frequency is increased or decreased, can be selected by controlling the bias signal provided to the MZC 104.
It is to be noted that the operation of the conventional optical SSB modulator is described in detail in, for example, [Tetsuya Kawanishi, Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08)] and [Higuma et al., “X-cut lithium niobium optical SSB modulator, Electron Letter, vol. 37, 515-516 (2001)].
In the conventional optical SSB modulator, the bias signal has been adjusted as follows in order to obtain a phase of light in the arm of each MZ waveguide described above. Namely, amplitudes (voltages) of the signals supplied to the MZA 102 and the MZB 103 are slightly adjusted so that the output from the MZC is minimized. Thereafter, modulation signals are impressed to the MZA 102 and MZB 103, and an amplitude of the bias signal supplied to the MZC is slightly adjusted so that unnecessary contents included in the output lights are minimized. Such operations were repeated.
However, in some cases the bias signal cannot be optimally adjusted due to an influence of an initial state of the MZC 104. Also, there are cases in which the output from the MZC 104 becomes 0 even when the optical phase in both arms of the MZA 102 and MZB 103 are not mutually different by 180°. Therefore, there has been a problem that the phase of light cannot always be appropriately adjusted with the conventional adjustment method. Also, when adjusting the bias signal supplied to the MZA 102 and MZB 103 in a state where a signal is not supplied to the MZC 104, there has been a problem that the bias signal cannot always be appropriately adjusted due to an influence of potentials or the like of the MZC 104, as described in Shimotsu Shinichi, Masayuki Izutsu, “LiNbO3 optical SSB modulator for next-generation communication”, Optical Alliance, 2000.7. pp. 27-30.